1. Field of the Invention
The present invention relates to an apparatus and a method for detecting the intensity and frequency of weak terahertz waves.
2. Description of the Related Art
In the present invention, the term “terahertz waves” means electromagnetic waves whose frequency is in the range of 1 to 50 THz (where, 1 THz=1012 Hz), i.e., whose wavelength is in the range from a 0.006-0.3 mm, corresponding to or belonging to submillimeter-wave region to the far-infrared region. In the following description, the “terahertz waves” are referred to as “terahertz light.”
The terahertz light is expected to be applied in a wide range of fields, ranging from basic science fields such as radio astronomy, materials science, and bimolecular spectroscopy to practical fields such as security, information communication, environment, and medical care.
However, since the terahertz light is electromagnetic waves the frequency band of which is interposed between light (5×1013 to 1015 Hz in frequency), such as near-infrared rays, visible light, and ultraviolet rays, and radio waves (103 to 1012 Hz in frequency), there is a problem that existing techniques, such as optics and electronics, cannot be applied as they are.
Various detectors for detecting terahertz light have been already proposed. Among them, Non-Patent Documents 1 and 2 disclose detectors capable of detecting very weak terahertz light having an intensity of a few fW (10−15 W).
In addition, Patent Document 1 discloses a detector capable of detecting the frequency of terahertz light.
In the meantime, “graphene” is an atomic monolayer of two-dimensional carbon crystal, having carbon atoms in each chain arranged in a hexagonal or honeycomb shape. In recent years, the unique electronic properties of graphene have attracted attention as reported in Non-Patent Documents 3 and 4, and already proposed in Patent Document 2.
Non-Patent Document 1 describes detecting detect terahertz light by using carbon nanotubes on a silicon substrate on which a silicon oxide film is formed. Non-Patent Document 2 teaches a terahertz light detector using superconductivity. Non-Patent Document 3 teaches a related technique of the present invention.
Patent Document 1 aims to obtain spectra having good S/N ratios in high frequency resolution measurements. To this end, as shown in FIG. 1, a detector body 51 disclosed in Patent Document 1 has a substrate 53, and a detecting element section (i.e., a portion of gap g between metal layers 55 and 56) including an optical switch element formed on the +Z-side surface of the substrate 53. A member 60 having nearly the same refractive index as that of the substrate 53 is provided on the −Z-side of the substrate 53 so that no terahertz pulsed light reflecting surface will be formed between the −Z-side surface of the member 60 and the +Z-side surface of the substrate 53. The shape of the −Z-side surface of the member 60 and the thickness of the member 60 are so set that light reflected on the +Z-side surface of the substrate 52 among the terahertz light incident from a certain area of the −Z-side surface of the member 60 and converged in the vicinity of the area (effective area) of the gap g will not substantially enter the area of the gap g after being reflected on the −Z-side surface of the member 60 or will enter the area of the gap g only after being reflected on the −Z-side surface of the member 60 twice or more.
Patent Document 2 relates to a graphene transistor and a manufacturing method thereof, aiming to provide a transistor using a graphene formed by a growth method for normal stabilized carbon nanotubes. To this end, as shown in FIG. 2, the graphene transistor disclosed in Patent Document 2 is made up as follows: Graphene 73 formed at a front edge of carbon nanotubes in the growth process thereof is stuck on a substrate 71 through an insulator 72 having adhesive properties. The graphene 73 is provided to serve as a channel, where a source electrode 74 is formed at one end of the channel, and a drain electrode 75 is formed at the other end of the channel, while providing a gate electrode 76.
[Non-Patent Document 1]
T. Fuse, et al., “Coulomb peak shifts under terahertz-wave irradiation in carbon nanotube single-electron transistors,” Applied Physics Letters 90, 013119 (2007).
[Non-Patent Document 2]
C. Otani, et al., “Direct and Indirect Detection of Terahertz Waves using a Nb-based Superconducting Tunnel Junction,” Journal of Physics: Conference Series, vol. 43, pp. 1303-1306 (2006).
[Non-Patent Document 3]
M. L. Sadowski, et al., “Landau Level Spectroscopy of Ultrathin Graphite Layers,” PHYSICAL REVIEW LETTERS, PRL 97, 266405 (2006).
[Non-Patent Document 4]
Z. Jiang, et al., “Infrared Spectroscopy of Landau Level of Graphene,” PHYSICAL REVIEW LETTERS, PRL 98, 197403 (2007).
[Non-Patent Document 5]
S. Masubuchi, et al., “Observation of Half-Integer Qyantum Hall Effect in Single-Layer Graphene Using Pulse Magnet,” Journal of the Physical Society of Japan, Vol. 77, No. 11, November 2008, 113707.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2003-232730 titled “TERAHERTZ LIGHT DETECTOR.”
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2008-205272 titled “GRAPHENE TRANSISTOR AND MANUFACTURING METHOD THEREOF.”
The terahertz light detector disclosed in Non-Patent Document 1 uses terahertz response of electrons captured at an impurity level in a silicon dioxide film. Therefore, the carbon nanotubes cannot be placed in desired positions with respect to the impurities during manufacturing of the detector. Further, since sharp wavelength selectivity is not provided at the impurity level, the frequency of terahertz light cannot be measured.
The terahertz light detector disclosed in Non-Patent Document 2 requires ultralow temperatures of 0.3 to 0.4 K to obtain high sensitivity, and hence it requires use of an expensive, large-scale helium-3 cryostat.
In the terahertz light detector disclosed in Patent Document 1, since terahertz light is absorbed by the member 60, very weak terahertz light having an intensity of a few fW (10−15 W) cannot be detected.
The conventional terahertz light detectors further have a problem that the frequency range of detectable terahertz light is narrow.